This may seem silly, but there are 3 little spring loaded catches on the spindle. If any one of them hasn't completely "clicked" you can get this message. Make sure the disc is seated all the way in the player and try again. Don't be afraid to press it kind of hard. If it clicks, you know you've done it. The same thing happened to me, and it *looked* like it should be working, but I was just being too gentle with it.
Hopefully, that's all it is.
Dan Taylor, August 2006
